Walltime required, but possible different time format required for LSF vs Slurm

[hagertnl]

The "walltime" replacement is required by the OTH, but the time format between LSF and Slurm can vary. For example, 1 hour in Slurm is 01:00:00, while it is 01:00 in LSF. I think it may be conducive to remove "walltime" from the required parameters list, and let developers choose do do things like defining walltime_lsf and walltime_slurm with the correct formats, and writing their batch scripts in such a way that they pick up the right variable.